Endochondral ossification is a pivotal event in skeletal development and in adult skeletal regeneration. In both, osteoclasts and chondroclasts erode hypertrophic cartilage, hypertrophic chondrocyte die, blood vessels grow in followed by recruitment of osteoprogenitor cells and bone and bone marrow cavity formation. These events require extracellular proteolysis and are essential for normal bone formation and repair. The goal of this proposal is to elucidate the molecular and cellular mechanisms, controlled by matrix metalloproteinases (MMPs), that co-ordinate the events in endochondral bone formation and regeneration. In the previous funding period we showed that proteolysis by MMP-9 is a critical determinant of the events in endochondral bone formation. MMP-9 and vascular endothelial growth factor (VEGF) act in overlapping pathways to regulate the program of hypertrophic cartilage remodeling, angiogenesis and bone formation in growth plate development and skeletal regeneration. We now propose to use genetic, molecular and cell Diological approaches to further characterize how these two MMPs, each separately and together, regulate the program of endochondral bone development. We will use targeted and conditional mutations in mice to evaluate the relative roles various cell types expressing MMP-9 and MMP-13 (collagenase-3) to cartilage emodeling, osteoblast and osteoclast function, vascular recruitment and skeletal remodeling, using morphometric and biochemical approaches coupled with cell and organotypic cultures. From these studies we will be able to determine the steps in skeletal growth plate morphogenesis. We will focus on terminal differentiation of hypertrophic chondrocytes, extracellular matrix remodeling, regulation of vascular recruitment and bone remodeling. We will then use the MMP mutant mice and unstable and stabilized fractures as tools to evaluate how the mechanical environment determines the regenerative response during in skeletal regeneration in wild type and mutant mice. We will then build on these analyses to determine the physiologic substrates of the MMPs during development and skeletal repair. These studies will give us insights into novel and important functions for the MMPs, extracellular matrix proteins and angiogenic regulators in endochondral bone formation. The endochondral ossification processes are misregulated in osteparthritis and recalcitrant hard tissue lesions. A fuller understanding of the central role of angiogenesis gjnd its regulation by proteolysis in these processes may have implications for therapeutic interventions and the development of therapy for certain types of non-healing fractures.